1. Field of the Invention
The present invention is broadly concerned with an improved extruded animal feed having a protein content which is highly water soluble, to meet the needs of certain animals (e.g., shrimp and piglets) which require soluble protein. More particularly, the invention is concerned with such extruded feeds, and a method of preparation thereof which incorporates the advantages of extrusion cooking while at the same time maintaining the high level of soluble protein required.
2. Description of the Prior Art
Many types of animal feeds are processed by extrusion cooking. For example, virtually all dry chunk-type dog foods are extrusion processed, as are numerous other types of livestock feeds. In normal extrusion processing, the selected ingredients for the feed are mixed with water, preconditioned by agitation and heating, and then extruded. The latter involves passage through an elongated extruder barrel equipped with an internal, axially rotatable helically flighted screw and an endmost extrusion die. During passage through the barrel, the combined ingredients are subjected to increasing levels of temperature, pressure and shear. As the cooked product emerges from the extrusion die, it is conventionally cut to a desired particle size. Downstream of the extruder, feed products of this type are normally dried to enhance their storage and handling characteristics.
As a consequence of this normal extrusion processing, the starch fraction of the feed is normally highly or completely cooked (gelatinized), and the protein fraction thereof is likewise largely denatured and rendered insoluble. In the case of many animal feeds, this type of extrusion processing is a perfectly adequate answer, i.e., the intended animals can digest and obtain nutritional benefits from the processed feeds.
It has been discovered, however, that certain types of animals have difficulty in digesting denatured and insolubilized protein. For example, shrimp and piglets are known to require soluble protein for best growth and health. Therefore, conventionally extrusion processed feeds having substantially denatured protein fractions are not entirely suitable for these animals. Nevertheless, the ease of processing, storage and use attendant to extrusion processed feeds makes it desirable that feeds of this type be provided.
It is also known that certain types of vitamins are relatively heat-sensitive, and that exposure of these vitamins to high temperatures tends to degrade or deactivate them. For this reason, it is sometimes difficult to extrusion process vitamins as a part of feeds, while retaining full vitamin potency. The same problem obtains with respect to other heat-sensitive recipe ingredients, such as flavors, pigments and medicaments.
There is therefore a decided need in the art for an improved extrusion process which can yield extruded feeds having a high degree of protein solubility and vitamin potency coupled with the product integrity, storability and use characteristics of conventionally extruded feeds.